The present invention relates to heat curable silicone rubber compositions and more particularly the present invention relates to flourosilicone heat curable silicone rubber compositions.
Heat curable silicone rubber compositions are well known. Generally such compositions comprise a diorganopolysiloxane gum having a viscosity varying from 1,000,000 to 300,000,000 centipoise at 25.degree. C., optionally a filler which is preferably a silica filler and a curing catalyst. The curing catalyst preferably is a peroxide catalyst. In the composition, there should be preferably utilized a polysiloxane process aid which facilitate the mixing of the filler in the diorganopolysiloxane gum and prevent it from structuring. There may be utilized other additives in the compositions such as heat flame retardant additives, heat aging additives, and self-bonding additives such as the self-bonding additives disclosed in De Zuba et al. U.S. Pat. No. 3,730,932 which is hereby incorporated by reference. An example of flame retardant additive is a platinum additive of Nobel et al. U.S. Pat. No. 3,514,424 which is hereby incorporated by reference.
It should be noted that a diorganopolysiloxane gum preferably contains some alkenyl groups which are preferably vinyl groups so as to ease the cross-linking and curing of the polymer. Fluorosilicone substituted heat curable silicone rubber compositions are also well known as disclosed in Brown U.S. Pat. No. 3,179,619. Other such compositions are produced by taking fluorosilicone substituted cyclopolysiloxane and heating them at elevated temperatures in the presence of an alkali metal hydroxide catalyst which is preferably potassium hydroxide in the presence of small amounts of chain-stoppers which may be water or low molecular silanol terminated diorganopolysiloxane polymers or may be aliphatic alcohol chain-stoppers, see for instance the disclosure of Bluestein Ser. No. 170,272. Such polymers are also disclosed to be produced with low molecular weight vinyl terminated chain-stoppers as for instance disclosed in the Patent Application of E. Robert Evans Ser. No. 071,152, filed Aug. 30, 1979, abandoned which is hereby incorporated by reference.
It should be noted that all the foregoing Patent Applications and Patents disclosed in this document are incorporated by reference in the present case. Thus utilizing a low molecular weight vinyl terminated fluorosilicone substituted copolymer as a chainstopper which is reacted in the presence of a sodium hydroxide catalyst with a fluoro substituted cyclotrisiloxane, it was possible to obtain diorganopolysiloxane gums of high molecular weight in a period of time varing anywhere from 3 minutes to 4 hours.
As disclosed in the copending case of Evans Ser. No. 253,282 filed Apr. 9, 1981 it was possible to obtain diorganopolysiloxane gums of a high viscosity, a viscosity varying from anywhere from 1,000,000,000 to 300,000,000 centipoise at 25.degree. C. where the 3,3-difluoro-2-propenyl or aliphatic unsaturation does not exceed 0.70 weight percent. It was found in such gums where the aliphatic unsaturation does exceed 0.7 weight percent that the resulting silicone elastomer formed from such gums had good strength properties and especially high tear. It was also possible to utilize the process of the foregoing patents to produce flurosilicone copolymers in which the aliphatic type unsaturation did not exceed 0.70 weight percent. This was done by controlling the amount of unsaturation that was present in the cyclotrisiloxane that was utilized in the process by first controlling the amount of unsaturation that was formed in the cyclotrisiloxane by the process by which it was made, and also but not utilizing batches of cyclotrisiloxanes in which the vinyl unsaturation exceeds 0.70 weight percent.
It was possible to produce fluorosilicone substituted copolymers which were vinyl terminated by equilibrating fluoro substituted cyclotrisiloxanes with octaorganotetrasiloxanes in the presence of an alkali metal hydroxide catalyst at elevated temperatures, that is temperatures above 140.degree. C. There was produced a copolymer having from 5 to 40 mole percent of trifluoropropyl substituted groups. The method of producing these copolymers is disclosed in the copending Patent Application of Evans et al. Ser. No. 253,282 filed Apr. 9, 1981. These copolymers have had many desirable properties and produce addition cured compositions with good strength properties.
The base homopolymer as well as the copolymer were tested for utilization on gas masks. In the past, gas masks had been produced from a timethylsilicone polymer in which the lens was formed from transparent plastic and in which the frame supporting the lens as well as the straps was formed from a dimethylpolysiloxane gum. However, it was necessary to coat the dimethylpolysiloxane gum frame with various types of fluorosilicone compositions and coatings so as to make the frame impervious to nerve gas.
Dimethylpolysiloxanes are not impervious to nerve gas. However the complicated coating procedure that was utilized tended to increase the cost of the mask.
Accordingly it was highly desirable to produce the frame and the straps of the gas masks from a fluorisilicone composition of the proper impervious ability to nerve gas at a lower cost. The foregoing vinyl terminated fluorosilicone homopolymers and copolymers were tested for this application. However, it was found that they do not have as high strength properties as would be desired although they do indeed have good strength properties.
Accordingly it was highly unexpected by including two vinyl terminated polymers with a dramatically different vinyl-on-chain concentration that there could be obtained exceptionally high strength fluorosilicone heat curable silicone rubber compositions. It should be noted that high strength high vinyl blends of diorganopolysiloxane gums to produce nonfluoronated heat curable silicone rubber compositions are disclosed in Bobear U.S. Pat. No. 3,660,345. However the present fluorosilicone compositions were not known prior to the present time.
It is one object of the present invention to provide for a high strength fluoro substituted heat curable silicone rubber composition.
It is another object of the present invention to provide for a high strength heat curable silicone rubber composition which is produced by blending diorganopolysiloxane gums of different vinyl-on-chain concentrations.
It is yet an additional object of the present invention to provide a process for producing a vinyl terminated high organo vinylsiloxy fluoro substituted copolymer of a viscosity varying from 1,000,000 to 300,000,000 centipoise at 25.degree. C.
It it still an additional object of the present invention to provide a process for producing a high strength fluoro substituted heat curable silicone rubber composition.
It is a further object of the present invention to provide a gas mask in which the frame of the mask and the straps are formed from a high strength fluoro substituted heat curable silicone rubber composition which is impervious to gases. These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.